Why do attached girls do this?

Depends on two things: how many guys does this girl flirt with? (some women just like getting a reaction from guys. It's all innocent. And in no way would they act on it) & second how often does she do this? And what situations? - Sometime friendly and outgoing girls can seem very flirty, but they are just friendly not flirty. The best way to make this judgement is see how often, and how many guys she flirts with. They want to be the "relationship" girl with their boyfriend, but at the same time want to know if they 'still got it' -- are they still able to get other guys attention?

Sometimes girls do this so their boyfriend realizes that she can get anyone, anytime, anyday. Basically he should feel honored that the girl is around him in such an exclusive, 'serious' way. Honestly, I think it's stupid and mainly I think it's just a confidence booster. If your with a guy that genuinely appreciates you and cares for you -- then these kind of antics are not only stupid but hurtful for the other guy. Makes sense I hope!